Processing of Ribosomal RNA of HeLa cells in vitro. The formation of ribosomal RNA involves the step-wise cleavage of a large precursor RNA found in the nucleolus to smaller RNAs found in mature ribosomes. The precursor RNA is complexed with protein to form pre-ribosomal particles; they contain the complete sequence of mature ribosomal RNA plus 50 percent disposable sequence. Using such materials as substrate for an in vitro system, a nucleolus-bound ribonuclease has been identified which leads to the formation of particles which appear identical by acrylamide gel electrophoresis to various stages of ribosome maturation. The purification of this enzyme is proposed, along with a search for additional enzymes of RNA processing in nuclear fractions. Following purification of the nucleolar RNase, the reaction it carries out will be characterized with emphasis on cleavage sites, and possible role of the dispensible region of the precursor. Finally, pre-ribosomal particles will be isolated from cells under different conditions known to alter RNA processing (antibiotic treatment, amino acid starvation), and their activity in the in vitro systems will be tested to determine what role substrate modification plays in the regulation of ribosome formation. II. Temperature Sensitive Processing Mutants. A temperature-sensitive mutant of Saacharomyces cerevisiae appears defective in nuclear processing of RNA or nuclear-cytoplasmic transport. Studies of the kinetics of formation and disposition of different messenger and ribosomal RNA precursors should yield an exact characterization of the lesion. These studies on RNA processing in eucaryotic cells should yield a greater understanding of what role the regulation of such processing plays in the control of cellular growth.